Zeolites are crystalline or quasi-crystalline aluminosilicates constructed of repeating TO4 tetrahedral units with T being most commonly Si, Al or P (or combinations of tetrahedral units). These units are linked together to form frameworks having regular intra-crystalline cavities and/or channels of molecular dimensions. Numerous types of synthetic zeolites have been synthesized and each has a unique framework based on the specific arrangement of the tetrahedral units. By convention, each topological type is assigned a unique three-letter code (e.g., “SZR”) by the International Zeolite Association (IZA).
Zeolites have numerous industrial applications, and zeolites of certain frameworks, such as CHA, are known to be effective catalyst for treating combustion exhaust gas in industrial applications including internal combustion engines, gas turbines, coal-fired power plants, and the like. In one example, nitrogen oxides (NOx) in the exhaust gas may be controlled through a so-called selective catalytic reduction (SCR) process whereby NOx compounds in the exhaust gas are contacted with a reducing agent in the presence of a zeolite catalyst.
Synthetic zeolites of the SZR topological type when prepared as aluminosilicate compositions are produced using structure-directing agents (SDAs), also referred to as a “templates” or “templating agents”. The SDAs that are used in the preparation of aluminosilicate SZR topological-type materials are typically complex organic molecules, which guide or direct the molecular shape and pattern of the zeolite's framework.
Generally, the SDA can be considered as a mold around which the zeolite crystals form. After the crystals are formed, the SDA is removed from the interior structure of the crystals, usually by heating in air, leaving a molecularly porous aluminosilicate material.
SUZ-4 zeolite was first reported by S.A. Barri, U.S. Pat. No. 5,118,483 (1992). In typical synthesis techniques ((1) Gao, S.; Wang, X.; Chu, W. The first study on the synthesis of uniform SUZ-4 zeolite nanofiber. Microporous and Mesoporous Materials 2012, 159, 105-110; (2) Gao, S.; Wang, X.; Wang, X.; Bai, Y. Green synthesis of SUZ-4 zeolite controllable in morphology and SiO2/Al2O3 ratio. Microporous and Mesoporous Materials 2013, 174, 108-116; and (3) Vongvoradit, P.; Worathanakul, P. Fast Crystallization of SUZ-4 Zeolite with Hydrothermal Synthesis: Part I Temperature and Time Effect. Procedia Engineering 2012, 32, 198-204), solid zeolite crystals precipitate from a reaction mixture which contains the framework components (e.g., a source of silica and a source of alumina), a source of hydroxide ions (e.g., NaOH or KOH), and an SDA. Such synthesis techniques usually take several days (depending on factors such as crystallization temperature) to achieve the desired crystallization. When crystallization is complete, the solid precipitate containing the zeolite crystals is separated from the mother liquor, which is discarded. This discarded mother liquor contains unused SDA, which is often degraded, and unreacted silica.
SUZ-4 has a needle-shaped morphology. (Lawton, S.L., Bennett, J.M., Schlenker, J.L. and Rubin, M.K., Synthesis and proposed framework topology of zeolite SUZ-4, Chem. Commun., 894-896 (1993)) and (Strohmaier, K.G., Afeworki, M. and Dorset, D.L., The crystal structures of polymorphic SUZ-4, Z. Kristallogr., 221, 689-698 (2006)).
Concerns have been raised about the use of aluminosilicates having needle-like morphology due to similarities with asbestosis. For example, erionite is a natural zeolite having an ERI framework type. The morphology of erionite has been classified as being: single crystals as hexagonal prisms terminated by a pinacoid with sizes under 3 mm. (IZA Commission on Natural Zeolites). It has been shown that exposure to erionite can result in a potential health hazard because, compared to other mineral particles, erionite has been shown to have greater pathogenicity than asbestos. (Michele Mattioli, Matteo Giordani, Meral Dogan, Michela Cangiotti, Giuseppe Avella, Rodorico Giorgi A. Umran Dogan, and Maria Francesca Ottaviani; Morpho-chemical characterization and surface properties of carcinogenic zeolite fibers; Journal of Hazardous Materials 306 (2016) 140-148) (Elizabeth A.Oczypok, Matthew S. Sanchez, Drew R. Van Orden, Gerald J. Berry, Kristina Pourtabib, Mickey E. Gunter, Victor L. Roggli, Alyssa M. Kraynie, and Tim D. Oury; Erionite-associated malignant pleural mesothelioma in Mexico; Int J Clin Exp Pathol 2016; 9(5):5722-5732.)
One potential method to address concerns related to the morphology of an aluminosilicate having a specific framework type is to develop a form having a different morphology while maintaining the same framework type. (U.S. Pat. No. 5,961,951A; Kennedy, C.L.; Rollmann, L.D.; Schlenker, J.L. Synthesis ZSM-48. 1999.) (U.S. Pat. No. 6,923,949 B1; Lai, W.F.; Saunders, R.B.; Mertens, M.M.; Verduijn, J.P. Synthesis of ZSM-48 crystals with heterostructural, non ZSM-48, seeding. 2005)
There is a need to develop new zeolites having the basic structure of known zeolites, where minor changes in the product morphology can affect one or more of the properties of the zeolite. In some cases, while minor changes in the morphology may not be discernable using commonly used analytical techniques, the catalytic activity of the modified zeolite may be improved relative to very closely related analogous zeolites. Unexpected improvements in the catalytic activity of such morphologically modified zeolites can allow for the compositions of exhaust gases from engines to meet various regulatory requirements.